Commonly assigned U.S. Pat. No. 4,386,129 describes a porous polymeric film having pores with polygonal cross sections and average pore diameters of about 3 to about 100 microns formed from a resinous polymer of propylene and the process for forming the films comprising the steps of forming a film containing beta-spherulites and selectively extracting the beta-spherulites. Such films have utility as filtration devices, raincoats, and tents. As contemplated by the patent, such films also can be subjected to post-processing steps such as calendering, lamination, or stretching, to improve or modify properties such as strength, thereby expanding their potential utility. For example, as with nonporous polypropylene films, stretching of such porous films in their longitudinal or transverse directions, or both, can be conducted to strength the films due orientation of polymer molecules. However, the fine interconnected pore structure of the porous films is susceptible to distortion on stretching such that desirable properties, such as waterproofness, may be sacrificed. As can be appreciated, it would be desirable to obtain the benefits of stretching in terms of increased breathability, higher tensile strength, improved drapability while maintaining a suitable combination of waterproofness, porosity and pore structure in the stretched films.
Various porous stretched polymeric films are known in the art. There are three categories of technology to produce porous, melt processed polymeric films in which a stretching step is involved. These three categories are processes for stretching films of neat, unblended polymers that do not contain fillers, other than typical stabilizing additives; processes for making films from blends of two or more polymers, or from a blend of a polymer with mineral oil or an organic salt in which the dispersed phase may be extracted with the film stretched either before or after the extraction; and processes for producing films made from a polymer that has been blended with a filler such as calcium carbonate or barium sulfate with the film stretched after casting without any extraction.
In the first category, a common method of producing such films involves drawing or stretching a crystalline, elastic starting film to about 10 to 300 percent of its original length as exemplified by U.S. Pat. Nos. 3,426,754; 3,558,374; 3,539,374; 3,551,363; 3,690,977; 3,801,404; 3,843,761; 3,839,240; and 4,138,459. This drawing operation is said to produce a microporous film ordinarily having elongated, slit-like pores with a pore size less than 0.5 micron although U.S. pat. No. 3,839,240 discloses a process for producing pores as large as 1.2 microns. U.S. Pat. No. 3,920,785 describes post-treating a stretched film with an organic solvent to increase the gas transmission rate of the film. U.S. Pat. No. 4,105,737 describes a process for producing a porous film by forming many fine cracks in a stretchable polymer composition having a fine phase separation structure, heating the polymer composition to its stretching temperature under tension so that the fine cracks do not disappear and enlarging the cracks by stretching the film at the stretching temperature. U.S. Pat. No. 3,839,516 describes a process for swelling a polyolefin film by immersing the film in a solvent such as toluene or benzene, stretching the polyolefin film in the swollen state and drying the film under tension to form pores. EP No. 0 256 192 describes a process for preparing a sheet of sintered polymer particles and then stretching this sheet on a tentering device to create a liquid permeable lace-like structure.
In the second category, U.S. Pat. No. 3,956,020 describes a process for dissolving a benzoate salt from a polymer article to form an ultrafine porous article, U.S. Pat. No. 4,076,656 describes a process to incorporate a water soluble liquid into a polymer and to extract the liquid with water, and in U.S. Pat. No. 3,607,793, a hydrocarbon liquid is extracted from a polymeric gel. U.S. Pat. No. 3,407,253 describes a process to form sheets from blends of polypropylene and an elastomer such as polyisobutylene which are drawn to create internal voids, thereby imparting some breathability. U.S. Pat. No. 3,969,562 describes a process of blending two crystalline polymers together, extruding a sheet, cold stretching the sheet to open up crazes and then hot stretching the sheet biaxially to increase porosity. U.S. Pat. Nos. 4,100,238 and 4,197,148 describe processes for blending two partially compatible polymers together, forming a sheet or film from the blend, immersing the film in a solvent for one of the components and extracting the component, drying the resulting film which has an interpenetrating pore structure to remove the solvent and biaxially stretching the film. According to the patents, the resultant material has a very high moisture vapor transmission rate and is not waterproof. U.S. Pat. Nos. 4,116,892, 4,153,751 and 4,289,832 describe a process to extrude incompatible polymer blends into a sheet and then stretching the sheet by drawing it over a grooved roller at low temperatures to generate porosity. EP No. 0 273 582A describes a process in which polypropylene is blended with mineral oil and a nucleating agent and a cast film is produced from the blend. The mineral oil phase separates as droplets within the polypropylene matrix and the droplets are removed by running the film through an extraction bath. The nucleating agent is said to act to reduce the size of the droplets and thereby reduce the size of the holes in the final product. The patent also mentions that the extracted film may be biaxially stretched although no description is given regarding the moisture vapor transmission rate of the resulting material. EP No. 0 258 002A describes a process in which a hydrophobic polymer such as polyethylene is blended with a hydrophilic polymer such as polyethylene oxide and a blown film is produced from the blend, the polyethylene oxide is crosslinked with UV radiation with uncrosslinked polyethylene oxide extracted with water, the film dried to produce a film containing pores filled by interconnecting plugs of polyethylene oxide. According to this publication, the moisture vapor transmission rates of the formed materials are typically 10,000 grams per square meter per 24 hours or greater.
In the third category, U.S. Pat. No. 3,844,865 describes a process in which blends of a polymer with an inorganic salt such as calcium carbonate are formed into films and then stretched uniaxially or biaxially to obtain high moisture vapor transmission rates. U.S. Pat. No. 3,376,238 describes a process in which blends of polyethylene with sugar, starch, and silica gel are prepared and cast into film with the film crosslinked with peroxide or radiation and then pore formers are extracted at elevated temperatures with the films optionally biaxially stretched before the extraction step. U.S. Pat. No. 3,725,520 describes a process to blend a polyolefin with a hard filler and a high boiling solvent which is formed into a sheet and the sheet is biaxially stretched with the solvent evaporated before or after the stretching. U.S. Pat. No. 4,705,813 describes a process to blend a polyolefin with barium sulfate and optionally a lubricant, cast the blend into a film and stretching the film uniaxially or biaxially.
Also of possible interest in connection with the present invention, U.S. Pat. No. 4,185,148 describes a process for producing a polypropylene film having a surface layer of beta-form crystals. In that process the film is extruded through a thermal gradient such that one side of the film cools at a much more rapid rate than the other thereby producing beta-crystals on the cooled side. The resulting film is then biaxially stretched to produce a film having a rough surface and which is air-tight. The patent discloses that unstretched polypropylene film which is prepared by melt-molding a resin containing beta-form crystallization nucleating agent, having beta-form crystal structure is stretched, a film having roughened uneven patterns on the film surface may be obtained.
While various stretched or stretched, porous films are disclosed in the patents and publications discussed above, none discloses or suggests stretching of porous polymeric films prepared by forming a film containing beta-spherulites and selectively extracting the beta-spherulites, to provide an oriented porous film with desirable increases in porosity, nor do they disclose or suggest porous films containing up to 20 weight percent cornstarch to enhance biodegradability and photodegradability properties.
An object of the instant invention is to provide a process for preparing oriented porous films of resinous polymers of propylene.
Another object of the instant invention is to provide a process for forming oriented films of polypropylene-based resin having cells with polygonal cross-sections which are about 5 to about 30 microns in diameter and interconnecting pores between the cells which are about 0.2 to about 20 microns in diameter.
Another object of the instant invention is to provide oriented polymeric porous films of polypropylene-based polymer with a thickness of about 0.005 to about 0.2 millimeters having cells of polygonal cross-section with average diameters of about 5 to about 30 microns and interconnecting pores between the cells which are about 0.2 to about 20 microns in diameter.
Another object of the instant invention is to improve biodegradability and photodegradability of polymeric porous films of polypropylene-based resin.
We have now found that the objects of this invention can be obtained by stretching in at least one direction to about 1.5 to about 7.5 times its original length in such direction and under controlled stretching conditions, a porous film comprising a polypropylene-based resin and having a moisture vapor transmission rate (MVTR) of about 2,500 to about 7,500 grams/square meter/24 hours and having cells with polygonal cross-sections which are about 5 to about 30 microns in diameter and interconnecting pores between the cells which are about 0.2 to about 20 microns in diameter. Advantageously, the resulting films exhibit improved strength relative to the unstretched film and, in addition, have cell and pore structures and porosity such that a MVTR of about 2,500 to about 7,500 grams per square meter per day are obtained. Further, stretching according to the present invention allows for preparation of films of desirable porosity and MVTR from unstretched films which themselves can be obtained under simplified extraction conditions, thereby improving efficiency of the overall process. In a particular embodiment, biodegradability and photodegradability of the films is enhanced by incorporation of cornstarch into the film.